PHOLAS. 195 
every five minutes a powerful and copious jet from both 
tubes, sometimes simultaneously, at others at imtervals, and 
that from the branchial tube in the Pholades is almost 
always accompanied by a strong ejection from the pedal gape, 
and also in the Veneres from the pedal aperture, though from 
the absence of a gape in their shells it is not so visible. The 
periodic times of the in- and out-flux, of whichever character 
it may be, as the animal becomes exhausted, are more and 
more prolonged ; they are only in vigour for twelve hours. 
What is the object of these copious and regular receptions 
and expulsions of water? Will not every reasonable person 
acknowledge that they can only serve for branchial purposes 
—the receptions, to administer water to the gill-laminz, and 
the expulsions to remove it when effete by the contraction of 
the adductors of the valves and siphonal retractors? In cor- 
roboration of the above, I particularly refer to the Rev. James 
Bulwer’s account of the Isocardia cor, published in the 
‘Zoological Journal,’ vol. i. p. 258. Messrs. Alder and 
Hancock cannot controvert these facts, and therefore in rela- 
tion to my theory say, “This is, however, a special case 
having nothing to do with the regular branchial currents, as 
has before been pointed out to Mr. Clark.” 
My opponents may find their special case a general law, 
and their system of regular branchial currents an illusion. 
They, having discovered that no ascertained communication 
—(this is a condition of my theory|—existed between the 
branchial and anal chambers, thus express the fact :—‘“‘ We 
certainly find no opening between the foot and the gills, nor 
between the gills and the mantle; and in consequence of 
their favourite doctrine being in jeopardy, they “found it 
necessary to make a careful examination of the anatomical 
structure of these animals,’ and have informed us of the 
discovery of a channel, by declaring the gill-laminz and their 
interbranchial tubes permeable, on which—to them a most 
important fact, if true—they emphatically observe, ‘ Thus 
in an instant the secret was explamed; the currents commu- 
nicate through minute openings in the lamine of the gill- 
plates.” 
02 
